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BIG PHYSICS, BIG QUESTIONS –

Hubble’s new night-vision unveils stars

By Eugenie Samuel

The first stunning pictures have been released from the Hubble space telescope’s revived night-vision camera, and they already have astronomers scratching their heads.

In March, the Hubble Space Telescope was upgraded with a new optical camera, the Advanced Camera for Surveys (ACS), with vision 10 times better than its predecessor. But the servicing mission also resurrected an infrared camera called Nicmos by giving it a new cooling system.

When infrared radiation hits a telescope’s optics they warm up, and this can generate an infrared glow of its own that drowns out the signal. To prevent this, the camera has to be cooled to -200 °C, much colder than the -90 °C at which the rest of Hubble operates.

Trials with the original solid-nitrogen cooling system went awry in 1998 because the operating temperature was precariously close to the point at which the nitrogen sublimes into gas. So the team eventually settled on a neon gas refrigerator.

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“Things were quite slow at the beginning but in the end everything’s worked out,” says Nicmos scientist Edward Cheng of NASA’s Goddard Space Flight Center in Maryland. The upgrade means Hubble can now image infrared light with a wavelength of up to 2 micrometres, compared with a previous limit of 1 micrometre, only just beyond the red end of the visible spectrum.

Stars unveiled

Many of Hubble’s optical pictures look so stunning because the telescope is able to resolve the huge clouds of dust surrounding distant galaxies and nebulae. But the dust also obscures information about these objects’ internal structure.

“It’s like looking into a puff of smoke,” says Cheng. Nicmos can see right through the clouds, however, because the longer wavelengths of infrared light are not scattered so easily by interstellar dust particles. “We’re really excited about the infrared being able to add a lot of information to the optical images,” Cheng says.

The new camera is already living up to its promise. When astronomers used it to look at the Cone Nebula, they were able to see a group of small stars that were not visible in pictures taken by Hubble’s optical camera.

But it is not clear yet whether the stars are part of a star-forming region inside the nebula, or whether they are behind it. Further observations should clear this up, Cheng says. In other newly released images the internal structures of distant galaxies have also become visible for the first time.